The present invention relates to seagoing vessels and, in particular, to a hull construction which increases the propulsion efficiency of such vessels.
Among the goals sought to be reached in the design of large boats, such as support vessels for offshore petroleum operations for example, is the maximization of propulsion efficiency in order to reduce fuel costs and increase the bollard pull. One factor having major impact upon propulsion efficiency is the configuration of the hull of the vessel. A typical hull includes a bow formed by side walls which diverge rearwardly and eventually assume a generally parallel relationship. It is conventional to provide a bulb at the bow to reduce hull resistance during a ballasted condition of the ship (e.g., see Taylor et al. U.S. Pat. No. 3,946,687 issued Mar. 30, 1976 and Weicker U.S. Pat. No. 3,455,262 issued July 15, 1969). The transition regions in which the rearwardly diverging portions of the side walls assume a generally parallel relationship occur at some distance aft of the bow and may be considered to form "shoulders", the presence of which creates turbulence as the vessel advances through the water. Such turbulence increases hull resistance and reduces the propulsion efficiency of the ship. The amount of turbulence which is created is a function of the abruptness of the transition zone, i.e., a more abrupt transition zone produces more turbulence than a gradual or streamlined transition zone. On the other hand, a highly streamlined transition zone results in a blunted bow shape, which shape produces more turbulence at the bow than does a pointed bow shape.
Therefore, the design of a bow to enhance propulsion efficiency involves mutually conflicting considerations which necessitate that compromises be made.
The hull design at the stern can also significantly affect the propulsion efficiency. For example, by inclining the underside of the hull rearwardly upwardly at the stern, the wake produced by the ship can be reduced, thereby lessening the amount of hull resistance. However, such an inclining of the hull underside can adversely affect the propeller performance. That is, it is desirable to maximize the water velocity traveling to the propellers in order to enhance the propeller efficiency. By inclining the underside of the hull at the stern, however, the water approaching the propellers encounters a gradually increasing area and can undergo a considerable loss of velocity.
With further regard to the propellers, it is noted that the propeller shafts are often encased within skegs or housings which diverge rearwardly from the backside of the hull. The presence of such skegs can influence the water flowing toward the propellers, e.g., by creating efficiency-robbing turbulence, in such manner as to reduce the propeller performance and create vibrations in the propeller.
Another factor which can adversely affect propeller performance is propeller vibration induced by turbulence and/or instability of the engine and propeller shaft bearings, which phenomena can occur to some extent in vessels. For example, vibrations in the propeller shaft can be caused by relative flexing between various parts of the hull. That is, it is common to mount the engine upon one section of the hull, and the propeller shaft suport bearings on another section. Although the hull is typically formed of steel, it is subject to flexing during vessel travel. As a result, the propeller shaft, engine and bearings can experience relative displacements, whereby undesirable stresses are imposed on the propeller shaft that can lead to damage of the bearings and excessive vibration of the propeller shaft.
It is, therefore, an object of the present invention to minimize or obviate problems of the types discussed above.
A further object of the invention is to increase fuel efficiency and pull capacity.
Another object is to minimize the amount of turbulence created at the bow and transition zones of the hull of a seagoing vessel.
An additional object of the invention is to maximize the propeller performance of a multi-propeller vessel.
A further object of the invention is to optimize the behavior of water flowing to the propellers from the standpoint of propeller efficiency.
Yet another object of the invention is to minimize vibrations occurring in the propeller, propeller shaft, and engine.
A further object is to stabilize the engine and propeller shaft against flexing movements of the hull.